Projectors and other image display systems based on digital light modulation techniques, such as digital micromirror devices (DMDs), use a lens system to project a pixelated object from the surface of a DMD-based display onto a projection screen. The surface of the digital light modulating display is typically located at or near an optical focal plane (hereinafter called an object plane) on an object side of the lens system. A projection screen is typically located on a projection side of the lens system. In this typical configuration, the lens system maps spatial points of light at the object plane to outgoing light rays at the projection side of the lens system. The outgoing rays impinge on a projection screen to display an image of a pixelated object located at the display surface of the digital light modulating device. The lens system is characterized by an overall focal length (FL), which, for reasons described below, is preferably short.
One disadvantage of conventional short FL lens systems is that the back focal length (BFL) is not large enough. Conventional lenses with a short FL have a short BFL. For a variety of applications, it would be useful to have a BFL that is large enough to place additional optical components in the light path between the object plane and the lens system. Simultaneously, it would be advantageous for a lens system to have a short FL to maintain a wide field of view and therefore a small throw ratio (projection distance/projected image width). In terms of a projection system, the throw ratio is proportional to the overall focal length FL of the projection lens system divided by the size of the object at the object plane. For a given object size, conventional lens systems do not provide a sufficiently large BFL while maintaining a short FL in order to provide: additional space for optical elements, a large field of view, and a small throw ratio.
Another disadvantage of conventional lens systems for projection applications is that they do not use spherical optics. Conventional lens systems often employ a number of aspherical lenses to correct various deleterious optical effects caused by light propagation through lens materials. These effects include spherical and chromatic aberration. The optical effects that are important for digital light modulating and other projection displays are associated with a number of optical measurements, such as low distortion, high telecentricity, low lateral color, and a high modulation transfer function (MTF). Digital light modulating display systems normally use a lens system comprised of aspherical lenses in order to satisfy the performance requirements for these optical measurements. Moreover, these conventional lens systems may use exotic lens materials. Lenses made from exotic materials, as well as aspherical lenses, are typically more expensive to develop and produce. It would be advantageous for a lens system to be able to employ less expensive spherical optics made of standard optical glasses or similar materials while satisfying strict performance requirements, providing sufficient correction for deleterious optical effects digital light modulation projection applications.
The present invention was developed in light of these and other drawbacks.